The third proportional to 9 and 15 is:

Let the third proportional to 9 and 15 be x.
By definition of a third proportional in a continued proportion:

9:15::15:x
This can be written in fraction form as:

15
9
​
=
x
15
​

Now, solve for x:

9×x=15×15
9x=225
x=
9
225
​
=25
Thus, the third proportional is 25.